As described above, ${time} placeholder sets time with Time.at(time).strftime("%Y-%m-%d %H:%M:%S").
This handles the time with fluentd server default timezone.
If you want to use the specific timezone, you can use the include_time_key feature.
This is useful in case fluentd server and mysql have different timezone.
You can use various timezone format. See below.
http://docs.fluentd.org/articles/formatter-plugin-overview